The purpose of this project is to explore the possibility that low density lipoprotein (LDL), once taken up by cells, can be degraded and transformed into unesterified cholesterol-rich lipid particles similar to those that we have previously identified and isolated from atherosclerotic lesions (see report Z01 HL 02826-07 EA). Two model systems, cultured fibroblasts from patients with type C Niemann-Pick disease (NPC) and livers of NCTR-BALB/c mice, were chosen for initial studies of cellular production and accumulation of unesterified cholesterol-rich lipid particles. Both models are autosomal-recessive lipid storage disorders with defects in intracellular cholesterol processing resulting in accumulation of unesterified cholesterol rather than esterified cholesterol the usual storage form of excess cellular cholesterol. Unesterified cholesterol-containing inclusions accumulate within cultured NPC fibroblasts during incubation with LDL. These inclusions appeared to be multilamellar liposomes with a hydrated density between 1.03 g/ml and 1.06 g/ml. The liposomes contained most of their cholesterol in an unesterified form (>90%) with a 1.5:1 molar ratio of unesterified cholesterol to phospholipid. NPC fibroblasts and normal fibroblasts incubated with lipoprotein- deficient serum plus LDL did not accumulate similar liposomes. This indicates that these unesterified cholesterol-containing liposomes were specifically derived from degradation of LDL and abnormal processing of cholesterol released from degraded LDL. Unesterified cholesterol-containing vesicles accumulated in livers of NCTR-BALB/c mice. These vesicles appeared to be uni- and multilamellar structures with a hydrated density between 1.04-1.07 g/ml. Most of their cholesterol was also in an unesterified form (>90%) with a 2.5:1 molar ratio of unesterified cholesterol to phospholipid. Studies carried out with both models should be helpful in elucidating the mechanism of formation of the unesterified cholesterol-rich lipid particles that accumulate within atherosclerotic lesions.